Comet Shoemaker-Levy 9 was originally "a rubble
pile composed of hundreds, if not thousands, of smaller
snowballs" no greater than 1.5 kilometers in diameter.
Observers will probably see a smaller explosion higher in
Jupiter's atmosphere than they now expect, University of
Arizona astronomer Willy Benz reports in the July 14 issue
of Nature.

Benz, an associate professor of astronomy at the UA
Steward Observatory, and former UA graduate student Erik
Asphaug, now with the NASA Ames Research Center,
collaborated in modeling the structure of Comet SL9. Benz
is in Switzerland to observe the impact from the Geneva
Observatory and possibly from the Observatoire de Haute-
Provence, France.

"All the predictions and calculations of the impact
itself hinge on knowing what is actually impacting Jupiter,"
Benz said in an email message received yesterday. "We
tried to use the closest approach of SL9 with Jupiter in July
1992 to constrain cometary models. What we found was
that the usual large, homogeneous dirty snowball model
would not work. By this, we mean it did not break-up in
21-plus pieces when it flew by Jupiter.

"We investigated further and found that the model
that worked best...the model that best fit observations...was
one in which the comet was initially a rubble pile
composed of hundreds, if not thousands, of smaller
snowballs. The diameter of the parent object depends a bit
on the initial rotation of the comet, but is on the order of
1.5 kilometers.

"Our model implies that each of the 21 or so
fragments is actually composed of a large number of
smaller entities. All these snowball clusters are loosely
bound by their own gravity. As a result, we believe the
impact will be higher in the atmosphere than anticipated by
most numerical simulations." Some simulations assume
that each fragment is an ice ball with a diameter of about
two kilometers, Benz said. "The parent comet being
smaller, we also anticipate a somewhat smaller explosion."

Benz said of the coming impact, "Even if this impact
is somewhat less spectacular than the media would like to
have it, I believe that potentially much can be learned about
the structure of comets and the circulation in Jupiter's
atmosphere. And the best of all, we are going to learn all
this for a lot less money than (the cost of) sending a
spacecraft!"